Recent cars have airbags that inflate to protect drivers or passengers in emergency. However, accidental inflation of the airbag may damage drivers or passengers. In order to prevent such erroneous inflation, the conventional airbag system includes multiple systems for determining when a car collision occurs. The multiple systems includes at least two system, that is, a main system and a safing (fail-safe) system.
Some safing systems carry out mechanical processing from sensing to switching of inflator firing circuits. In view of recent demands for lighter weight and smaller size, however, many safing systems carry out electric processing as main systems do. FIGS. 9 and 10 show the circuitry of such airbag systems S of cars. The main and safing systems of both airbag systems S carry out electric processing.
In FIG. 9, the airbag system S includes acceleration sensors 71 and 72, a firing circuit 2 and an airbag 9. The firing circuit 2 includes a high-side switch 21, a low-side switch 22 and a squib 8. Determining circuits 61 and 62 consist of A/D converters 611 and 621, microcomputers 612 and 622, and signal output circuits 613 and 623, respectively. The signals output from the acceleration sensors 71 and 72 are converted into digital signals by the A/D converters 611 and 621. The microcomputers 612 and 622 process the digital signals to determine whether the car has collided or not. When the car collides, the output circuits 613 and 623 output firing signals to turn on the high-side switch 21 and low-side switch 22, respectively, in the firing circuit 2. This causes a firing current to flow through the squib 8, firing the inflator to inflate the airbag 9. The circuitry shown in FIG. 10 is similar to FIG. 9.
In the airbag systems shown in FIG. 9, the collision determining circuits 61 and 62 of the main and sating systems are provided independently of each other, that is, built into separate electronic circuit substrates, so that the system S is more reliable. However, the independent circuit substrates require a larger area, so that the electronic control units of the system are larger in size, and they make it more difficult to reduce costs.
In the airbag system S shown in FIG. 10, the collision determining circuits for the main and safing systems are built into one electronic circuit substrate 61a in common, so that it is easier for the system S to be smaller in size and less costly. However, the common unit makes the main and safing systems more liable to fail at the same time, so that the airbag system S is less reliable.